Field of the Disclosure
Aspects of the disclosure relate generally to an optical sensing system and, more particularly, to using a birefringent multi-peak optical reference element in an optical sensing system.
Description of the Related Art
Birefringent optical fibers for sensing parameters such as pressure are known in the art. In a birefringent fiber, birefringence is typically caused by the geometrical asymmetry or by anisotropic stress affecting the propagating modes of the fiber. Typical fiber sensors, such as those made from standard optical communication fibers, have outer diameters in the range of 125 microns with optical cores of 7 to 12 microns.
A large diameter waveguide, having an outer diameter ranging from about 0.3 mm to 4 mm, can provide several advantages for use as an optical sensor. Large diameter optical waveguides have a core and a cladding, as do standard fibers. In fact, the core of a single mode large diameter optical waveguide is generally the same diameter as the core of a single mode standard fiber, typically 7 to 12 microns. However, large diameter optical waveguides are thicker and sturdier than standard fiber because of the substantial amount of cladding in the large diameter optical waveguides. For example, while a standard fiber typically has a diameter of 125 microns, large diameter optical waveguides range from 0.3 mm to about 4 mm in diameter, which mostly constitutes cladding. The relatively thick cladding of large diameter optical waveguides provides significant mechanical benefits over standard fiber. In addition, a large diameter optical waveguide does not require a protective buffer layer and, thus, reduces manufacturing complexity. A large diameter optical waveguide may also be referred to as a “cane” or “cane waveguide,” due to the higher rigidity of large diameter optical waveguides as compared to standard fiber.
For sensing applications, optical waveguides may include one or more Bragg gratings. A fiber Bragg grating (FBG) is an optical element that is usually formed by photo-induced periodic modulation of the refractive index of an optical fiber's core. An FBG element is highly reflective to light having wavelengths within a narrow bandwidth that is centered at a wavelength referred to as the Bragg wavelength (also known as the characteristic wavelength, λ0). Other wavelengths of the light pass through the FBG element without reflection. The Bragg wavelength depends on characteristics of the optical fiber itself, as well as on physical parameters (e.g., temperature and strain) that affect the optical period of the grating. Changes in the Bragg wavelength due to the physical parameters will result in changes in the wavelengths being reflected by the FBG element. Therefore, FBG elements can be used as sensors to measure such parameters. After proper calibration, the Bragg wavelength provides an absolute measure of the physical parameters.
In practice, the Bragg wavelength(s) of one or more FBG elements are often measured by sweeping light across a wavelength range (i.e., a bandwidth) that includes all of the possible Bragg wavelengths for the FBG elements and by measuring the power (intensity) of the reflected light over time. While FBG elements are highly useful sensors, an example application may entail measuring the Bragg wavelength with a resolution, repeatability, and accuracy of about 1 picometer (pm) or better. For a Bragg wavelength of 1.55 microns (μm), a measured shift of 1 pm in the Bragg wavelength corresponds to a change in temperature of approximately 0.1° C. Because of the desired accuracy of the Bragg wavelength determination, some type of reference wavelength measurement system may be included. Making the problem of determining Bragg wavelength more difficult is the fact that the measurement system is typically sensitive to gradients, ripples, and optical polarization in the filtered light source spectrum that can induce small wavelength shifts in the measured peak wavelengths. This leads to uncertainties in the measured Bragg wavelength.
FBG sensor systems may include a wavelength reference system to assist determining the Bragg wavelengths. Such reference systems may be based on a fixed cavity length interference filter, typically a fixed Fabry-Perot wavelength filter, and at least one reference FBG element. When the wavelength swept light is input to the fixed cavity length interference filter, the filter outputs a pulse train that represents the fringes/peaks of the optical transmission or of the reflection spectrum of the filter (e.g., a comb spectrum having constant frequency spacing). This wavelength reference system reduces problems associated with non-linearity, drift, and hysteresis. The reference FBG element can be used for identification of one of the individual interference filter comb peaks, which is then used as the wavelength reference, or for relative wavelength measurements between FBG sensor elements and the reference FBG element. Thus, the comb spectrum establishes a frequency/wavelength scale.
By calibrating both the comb peak wavelength spacing of the reference-fixed Fabry-Perot wavelength filter and the peak wavelength of the reference FBG element versus temperature, and by accurately measuring the temperatures of the Fabry-Perot wavelength filter and of the reference FBG element, the Bragg wavelengths of the FBG sensors can be accurately determined. Alternatively, the temperatures of the fixed Fabry-Perot wavelength filter and of the reference FBG can be stabilized using an oven or an ice bath, for example.